This is what I am doing:
Options --> Languages --> File Extension Manager --> Click on "New" button, type "pdf" in the Extension field, Select "Plain Text" in the Language field.
On the main window that opened, kept "Automatic" as Encoding; selected the "Use file association" checkbox and clicked on "Apply". Restarted the application and double-clicked on a PDF file from within SlickEdit file manager. This should have launched Adobe Acrobat since it's associated with the file of type PDF, but it did not work. The file was opened within SlickEdit -- as if it's a plain text file.
I also tried another thing. Within the File Extension Manager window, I unchecked the "Use file association" checkbox and instead typed "c:\Program Files\Adobe\Reader 9.0\Reader\AcroRd32.exe" "%1" in the "Open application with" field. This did not work either, as the PDF file was again opened within the SlickEdit editor.
Is there a step that I am missing? I am using version 184.108.40.206 on Windows XP. My PDF file association works from within Windows Explorer -- so file association is not the problem.